Hi --- Get quite a few good quality digital photo's of the damage along with your initial receipt for the guitar when it was bought, you will also need this quotation for the damage in writing from the reputable repairer.
Once you have those, I presume that you're intending claiming on your Parents house contents insurance cover, if so it will need your Parent to contact their insurer and ask for the necessary claim form or to start the claim procedure.
You would be wise to investigate the excess on your policy first before making any claim, as the amount of excess plus the effect on future years policies, may make placing any claim unworthwhile. !